Thermal storage arrangements (whether heated by exposure to microwave energy or otherwise) have been widely used for a variety of purposes. Those which are activated by microwave energy have been used, for example, as heat cushions. One such arrangement utilizes as a thermal storage unit an absorbent polyurethane foam substrate which is soaked with water and which is encased in a de-aerated bag. Water, a high specific heat substance, is the microwave-sensitive energy absorber in such a system. Upon exposure to microwave energy, the water absorbs the energy and heats up. The polyurethane foam homogenously spreads the water over a greater area. It also operates as an insulator, which retards dissipation of heat from the water.
A conventional system, utilizing as the thermal storage component a polyurethane foam soaked with water, generally comprises a foam seat cushion or the like with the thermal storage unit embedded therein. In preparation for use, the heat cushion is placed in a microwave oven and is exposed to microwave energy for several minutes. The unit then remains warm as the water cools and gives off its heat to the surrounding cushion.
A problem with such conventional arrangements is that even with the polyurethane foam insulation present to retard the dissipation of the heat, water, in its liquid state, is not a very good thermal storage material. When cooled, water will dissipate only sensible heat, which is the product of the temperature differential between the water and the area surrounding it, and its specific heat. Thus, to provide a large amount of heat, a large volume of water is needed.